In regenerative thermal oxidation systems (RTO) harmful compounds are oxidized without residue and the oxidation energy plus the heater energy is recovered by the ceramic heat exchanger and is further used in the process resulting into reduced auxiliary fuels requirements and lower operating costs. In direct thermal oxidation systems (TO) several waste gas streams, liquid or gaseous, with concentrations from lean to explosive are injected directly in to the combustion chamber and oxidized. The combustion chamber can be combined with a variety of other purification systems (e.g. scrubber unit, rotor, fixed bed adsorber, filter or a DeNOx system). The purified gas can be directed to down-stream heat recovery.